I have 20 completed castles, and they are taking up a lot of space on my board. Can I sell them or..

You can sell them. If you select one, a yellow box will appear around it. Then at the bottom of the screen will be a white box with text and a blue button with a $ sign. Hit that to sell. Just so you know, they dont sell for very much, so do expect a big payout from it.

Yes, highlight it and at the bottom there is a description and you will see a dollar sign. Click the dollar sign to sell it.